The quantity of heat generated from semiconductor elements that are to be mounted onto a wiring board or substrate has increased as the performance of the semiconductor elements has been enhanced. It is therefore desired that the wiring substrate is made of a material having a higher thermal conductivity and heat radiating performance. Hitherto, an alumina sintered body has been used as a wiring substrate material; however, the alumina sintered body does not have a sufficient thermal conductivity. Thus, studies have been made on the use of an aluminum nitride sintered body, which has a higher thermal conductivity.
In order to produce a wiring substrate using a nitride ceramic sintered body, a typical example of which is the aluminum nitride sintered body, it is necessary to form metal wiring on a surface of the nitride ceramic sintered body. Methods for forming the metal wiring include the thick film method in which a metal paste is applied, the thin film method in which a metal thin film is formed by vapor deposition, etc. For articles required to have, in particular, heat radiating performance, the thick film method is favorably adopted since the articles need a large quantity of electric current in many cases and the thickness of any film formed by the thin film method imposes a restriction onto the electric current permitted to flow in the film. However, in the thick film method, metal wiring made mainly of a high-melting-point metal, such as tungsten or molybdenum, is formed, so that the method has a problem that the resistance of the wiring is high.
It is conceivable that as the material for metal wiring, Cu, Ag or Au, or any alloy thereof is used to reduce the wiring resistance. A noble metal such as Au is expensive, and the use of only Ag as the material is disadvantageous from the viewpoint of cost. The use of only Cu as the material is disadvantageous from the viewpoint of the wiring resistance. Thus, in view of the balance among the cost, high availability, and an intentional reduction in the resistance of metal wiring, it is most desirable to use a mixture of Cu and Ag as the material for metal wiring. Cu, Ag and the like can be sintered at a lower temperature than the above-mentioned high-melting-point metals; therefore, the former metals have an advantage that the cost of energy in sintering the metals can be decreased.
Known examples of an industrial method for forming metal wiring by the thick film method include the co-firing method and the post-firing method in which a paste containing powder of a high-melting-point metal is used. The co-firing method is a method of printing a high-melting-point metal paste onto an aluminum nitride green sheet, and then firing the sheet, thereby attaining the sintering of the aluminum nitride, and the firing of the high-melting-point metal simultaneously. The method has a characteristic that although an intensely adhering metal layer can be formed, it is difficult to form a metal pattern with a high dimension precision because of shrinkage of the aluminum nitride which follows the sintering thereof. When Cu, Ag or the like is used as the wiring material, the co-firing method cannot be adopted since the sintering temperature of aluminum nitride is largely different from that of the metal paste.
The post-firing method is a method of applying a paste of a high-melting-point metal onto an aluminum nitride substrate sintered in advance, and then firing the metal. In this method, a dimension-precision-related problem as described above does not basically occur. It has been hitherto stated that the post-firing method does not easily make the bonding strength (adhesion strength) of a metal layer high. However, a post-firing method has been developed which is capable of forming a high-melting-point metal layer which adheres (onto a substrate) with a high bonding strength (see Patent Literature 1). However, a technique has not yet been industrially established in which a paste of a different metal capable of making the resistance of wiring lower, such as Cu or Ag, is used to form a metal layer on a substrate of a nitride ceramic sintered body by the post-firing method.
As a solution to this problem, Patent Literature 2 discloses an aluminum nitride substrate having an aluminum nitride sintered body, and an electroconductive metallized layer formed thereon and made of an alloy containing, as an essential component, at least one selected from titanium, zirconium and hafnium. Patent Literature 3 discloses a metallizing metal powder composition, for forming a metallized film on a ceramic substrate, which contains Cu and Ti powders as a main component, and contains at least one of Ag, Al and Zr as a secondary component, in which the content by percentage of the main component is from 90 to 99.5% by weight and that of the secondary component is from 0.5 to 10% by weight; and discloses a process for producing a metallized substrate by use of this metallizing metal powder composition.
However, these techniques have the following problems: i) the adhesion strength of the (resultant) metal layer is insufficient; i) the resistance of the metal layer is not made low as expected; (iii) the metal-platability of the metal layer is lowered because of surface-roughness of the metal layer; and iv) a pattern (of the metal layer) oozes so that the metal layer cannot cope with any fine pattern.
As a technique for solving the problem iii) of the surface roughness of the metal layer, Patent Literature 4 describes a metallizing metal- and metal-compound-powder composition which contains, as main components, a powder of at least metal selected from Cu, Ag, Au and Ag—Pd, and a metal hydride compound powder in which the metal hydride powder is at least one selected from Nb, V, Hf and Ta hydrides. The literature suggests that the reason why the smoothness of metal layers cannot be increased is that the powder of Ti therein cannot be made fine (paragraph [0009]).
Patent Literature 5 describes an aluminum nitride substrate having a metallized layer, the substrate being obtained by applying, onto a sintered aluminum nitride substrate, a paste containing a Ag—Cu alloy as a main component and titanium hydride as a secondary component, and then firing the resultant.